Silicone surfactants, or more properly, organosilicones exhibit unusual properties that account for their use in a large number of specialty applications. For example, many have excellent wetting and penetrating characteristics.
The term silicone denotes a synthetic polymer which contains a repeating silicon-oxygen backbone and has organic groups attached to a significant proportion of the silicon atoms by silicon-carbon bonds. In commercial silicones, most R groups are methyl, higher alkyl, fluoralkyl, phenyl, vinyl, and a few other groups substituted for specific purposes; e.g., hydrogen, chlorine, alkoxy, acyloxy, and alkylamino.
Commercially useful silicone products are usually made by the process whereby silica is catalytically reacted with an RCR group which is usually methyl chloride. Hydrolysis of the organochlorosilanes formed yield the siloxane structures which are the bases of many silicon products as outlined in the reaction scheme I ##STR4##
The three commercially important classes of silicone polymers include silicone homopolymers, silicon random copolymers, and silicone-organic (block) copolymers. Polydimethylsiloxanes (II) constitute by far the largest volume of homopolymers produced today. ##STR5##
Polydimethylsiloxane is usually the principal component of the random copolymers and the principal siloxane building block or component of most silicone-organic copolymers.
The molecular weight of the polysiloxanes is usually controlled by the chain terminating groups. The trimethyl-siloxy group from hexamethyl disiloxane (III) results in polymers that do not polymerize further by chain extension. ##STR6##
In fact, the first known silicones are the trimethylsiloxy-terminated siloxanes. The properties of these siloxanes are modified by substitution of the methyl groups on the silicon atom in the --Si--O--backbone by hydrogen, alkyl, phenyl, or organofunctional groups.
Structurally, organosilicones derive their a polar (hydrophobic) properties from the siliconbased rather than carbon-based moieties. Polar groups, such as ethylene oxide chains, can be introduced into copolymers to provide enhanced hydrophilic properties. Common silicone surfactants are derivatives of monomethyl and dimethyl silicone compounds which are conjugated with ethylene or propylene oxide chains (glycols) or with substituted aliphatic carbon moieties containing amino or carboxyl substituents.
As a result of the weak intermolecular forces and the very high flexibility and rotational freedom that exist on the backbone of these polymers, linear siloxanes have very low melting points, do not crystallize under ordinary conditions; and, in fact, many are liquid at room temperatures.
As mentioned above, silicones have an unusual array of properties. Chief among these are thermal and oxidating stability and physical properties little affected by temperature. Other salient properties include resistance to weathering, ozone, and radiation; low surface tension; high surface activity; good spreading power; and, when unmodified, chemical and biological inertness.
Nonionic surfactants are commonly used as agricultural adjuvants to improve the efficacy of pesticides such as herbicides, fingicides, growth regulators, biologicals, and micronutrients. The surfactants play several roles in these agricultural formulations. For example, as activator agents, they can enhance the biological effectiveness of a pesticide. As a compatibility agent, they can selectively reduce or eliminate undesirable chemical interactions of two or more agrichernicals in a formulation and/or improve homogeneity of, for example, fertilizer with other agrichemicals in the mixture. As wetting or spreading agents, via reduction of surface tension in aqueous solutions, they increase the surface area covered by a given volume of the agricultural formulation. This is especially important for the spreading of solutions on difficult to wet surfaces such as a waxy leaf cuticle. The nonionic surfactants also aid in the uptake of active ingredients into plant tissue through permeation of the cuticle, through defects in the surface, and in some special cases, through flooding of the leaf stomata. Urea, ammonium nitrate, diammonium phosphate, and diammonium sulfate are also often used as agricultural adjuvants to supply nitrogen to crops and often, serendipitously to enhance the biological efficacy of pesticide formulations.
Certain organosilicone compounds have been recognized as excellent agricultural adjuvants, because of their outstanding wetting characteristics, enhancement of foliar uptake, and unique ability to increase the overall bioefficacy of many pesticide formulations especially those formulations containing glyphosate as the primary active.
Most of the organosilicone adjuvants, however, have the distinct disadvantage of being liquids, pastes or soft waxes at ambient temperatures. Thus, they are extremely difficult to include uniformly in dry pesticidal formulations. Attempts to overcome this liquid problem have utilized various adsorbents such as clays or silicas as solid carriers. However, these solid carriers are not soluble in water, not biologically active, clog fine spray lines and nozzles, and increase nozzle wear. Often, the surfactants are heated prior to blending to ease handling by decreasing viscosity which may, in turn, have a negative effect on its or the blend components chemical stability. In addition to the difficulties encountered in attempting to obtain uniform distribution of liquid surfactant in a powdered or particulate blend, the resulting tackiness ofttimes results in masses of material sticking to the walls of the blending apparatus.
In view of the above, it would be highly desirable to be able to produce agriculturally useful, oganofunctional polysiloxanes in a dry state.
Clathrates, also referred to as inclusion complexes are single-phased solids consisting of two distinct components; with the molecule of one component being retained in closed cavities or cages provided by the crystalline structure of the molecules of the second component. The two components of a clathrate do not react chemically with each other, but the solid clathrates have sharp melting points and always show integral values for the molecular ratios of the two components.
In contrast, a more recently explored class of inclusion complexes also known as tube, channel or canal inclusion complexes or adducts, form needle crystals and exhibit a lack of conformity to the classical law of simple multiple proportions. The most widely known examples of these non-stoichiometric complexes are the channel adducts of urea-n-paraffin, and thioureabranched chain paraffins. The molecules of one component are bound together, usually by hydrogen bonds, to give rise to large tubular intertwining polymer networks in which the molecules of the second component may become entrapped, anchored, or stabilized. The compound which traps or encloses another molecule has become known as the host, and molecules which become enclosed are often called the guest molecules. Two unique properties of these channel inclusion complexes are that they do not exist in solution in exactly the same form as they do in the crystalline state and their constituents do not exist in the crystalline aggregates in ratios of exact whole numbers. Since the urea and thiourea complexes have a non-stoichiometric ratio between the guest and host molecules which are governed by crystalline dimensions, it is impossible for them to exist in solution in exactly the same form as they exist in the crystalline, solid state. They form only as continuous crystalline host lattices and, although they sometimes appear to lack conventional bonding, many of these complexes are quite stable. The host molecules molecularly encapsulate and thereby modify the apparent physical and chemical properties of the guest molecules. An unusual property of the organic channel adducts is that their stability depends in part upon a very exact fit within the tubular cavity or cavities which the host molecules can form; thus we are also dealing with substances which depend on the size and shape of the guest molecules for interaction.
Early work with n-paraffms found that urea and thioureas form the channel molecular inclusion complexes, that is, the urea and thiourea molecules form a hollow channel just large enough to accommodate the planar zigzag of the n-paraffm hydrocarbon molecule; essentially large interpenetrating helical spirals forming a nearly circular dimensioned, or hexagonal latticed channel with the hydrocarbon molecules at the center.
Urea by itself forms a tetragonal structure, however, a crystalline transformation to the hexagonal structure occurs when an inclusion complex is formed.
Among the straight chained hydrocarbons, n-hexane is the smallest member which has formed an inclusion complex with urea. In general, with any homologous series, the stability of the inclusion complexes, i.e., the ability to form a separable, dry precipitate, increases with the chain length of the guest molecule. Large end groups have a negative effect on the formation of channel complexes which often can be overcome by significantly increasing the length of the hydrocarbon chain being complexed.
A number of other n-aliphatic organic compounds, besides the straight paraffinic chains have been reacted with urea. Fatty acid series have been studied as well as inclusion complexes of the n-alcohols, esters, halides, diglycerides, dibasic acids, olefins, and many related normal aliphatic structures. Inclusion complex studies of homologous series involving maleate, fumarate, and fluorinated esters have been reported.
With each class of compounds or homologous series, there is a minimum chain length which is required for adduct formation. For n-paraffins, the minimum chain length is six carbons at room temperature and pressure, but under pressure and at lower temperatures, even propane can be made to react. There is no theoretical upper limit to the length of paraffim chains which will complex with urea. In fact, urea channel adducts have been formed by reaction with poly (ethylene oxide) polymers as high as 4,000,000 in molecular weight.
Clathrates are generally prepared by recrystallization and precipitation from solution. If the host is soluble in the guest component, the preparation is simple. Otherwise, it is necessary to use a common solvent which cannot be clathrated by the host. Water is typically the solvent of choice. Of course, in crystallizing solutions where the concentration of the guest component is low, stirring and slow crystallization are necessary to avoid depletion of the guest component at the site of crystallization after initial clathrate formation.
Radell and Hunt (J. Am. Chem. Soc. 80, 2683 (1958)) prepared urea inclusion complexes of three monoalkylsilanes and four dialkylsilanes and reported that the complexes were white crystalline solids with "the melting point of urea". They noted that although hexane is the shortest hydrocarbon molecule that will form a crystalline urea complex under normal conditions, neither amylsilane nor hexylsilane would form such a crystalline structure. Thus they concluded that although a single silicon atom per se in the backbone of a linear hydrocarbon chain does not prevent the formation of a urea inclusion complex, it has a destabilizing influence.
In 1993, R. Davis of ICI Surfactants presented a paper entitled Solid Adjuvants Based On Urea-Surfactant Adducts in which he proposed that certain agrochemical surfactants would be good candidates for urca complexation, i.c., for conversion into free flowing powders. Among those suggested were, polyethylene glycols; EO/PO block copolymers; and ethoxylated alcohols, acids, and nonyl phenols. Ethoxylated tridecyl alcohol was exemplified. Davis also proposed that once such urea-surfactant adducts were formed, other adjuvants could be added to change the adjuvant properties of the final product such as phosphate ester acidifying agents, ethoxylated silicone wetting agents, and various sticking agents.
L. C. Fetterly (Study of Kinetics and Equilibria of Urea-Fatty Acid and Related Complexes, Ph.D. Thesis, University of Washington, Seattle, 1950) suggested that linear silicon polymers probably do not form inclusion complexes because the chain diameter is too large. He, in fact, tried to prepare complexes of these polymers, as well as an inclusion complex of dichlorosilane from urea and thiourea and was unable to do so. Illustrative of the sensitivity of the inclusion complexes to the diameter of the guest molecule, Fetterly noted that whereas normally one can form a urea inclusion complex with an n-paraffm carbon chain of 6 or greater; a linear paraffin chain of almost 18 C-atoms in length is required to off-set the distortion caused by a single methyl group in the 2 position.
Since it is presently impossible in most instances to predict solid molecular crystalline structure a priori, the discovery of a new clathrate or clathratable material has been and still is a matter of chance.